1. Field of the Invention
The present invention relates to a switching regulator, and more particularly, to an improvement of a power supply voltage variation response characteristic of a switching regulator.
2. Description of the Related Art
A conventional switching regulator includes a transconductance amplifier error amplifier. The transconductance amplifier error amplifier compares a voltage generated by dividing a voltage at an output voltage terminal by dividing resistors with a reference voltage produced from a reference voltage circuit and amplifies a potential difference therebetween. A gain of the transconductance amplifier error amplifier is determined based on an output current and a transconductance amplifier error amplifier output resistance. The switching regulator includes an LC filter which is composed of a coil and an output capacitor, so a significant phase delay occurs near a cutoff frequency of the LC filter. When the gain is equal to or larger than 0 dB in a frequency region in which the phase is delayed by 180 degrees or more, abnormal oscillation occurs. Therefore, it is necessary to reduce the gain in a high-frequency region near the cutoff frequency of the LC filter (see “SII CMOS IC DATA BOOK 2004, Power Supply IC•MOS FET Part”, pp. 4-314, FIG. 12).
FIG. 3 is a block diagram showing a switching regulator. The switching regulator includes a switching regulator control IC 34, a power supply 29, an output driver transistor 30, a diode 31, a coil 32, and an output capacitor 33.
The switching regulator control IC 34 has the following structure and operates as follows. A voltage at an output voltage terminal 35 is divided by a dividing resistor 37 to sense an output voltage. The output voltage generated by the voltage division using the dividing resistor 37 is compared with a reference voltage 39 by a transconductance amplifier error amplifier 38 and a result obtained by the comparison is amplified by the transconductance amplifier error amplifier 38. An output terminal of the transconductance amplifier error amplifier 38 is connected with a transconductance amplifier output resistor portion 40. A voltage outputted from the transconductance amplifier error amplifier 38 is compared with a voltage outputted from a triangular wave generating circuit 42 by a PWM comparator 41 to generate a PWM waveform corresponding to the voltage outputted from the transconductance amplifier error amplifier 38. The PWM waveform outputted from the PWM comparator 41 passes through a buffer 43 and then is inputted to a gate of the output driver transistor 30. A gain of the transconductance amplifier error amplifier 38 is determined by “(output current from transconductance amplifier error amplifier 38)×(resistance of transconductance amplifier output resistor portion 40)”.
FIG. 4 is a circuit diagram showing an example of the transconductance amplifier output resistor portion 40 of the conventional switching regulator. Resistors 3 and 4 are connected in parallel. One end of each of the resistors 3 and 4 is connected with an output-resistance input terminal 1. The other end of the resistor 3 is grounded and the other end of the resistor 4 is grounded through a capacitor 5. The output-resistance input terminal 1 is connected with the output terminal of the transconductance amplifier error amplifier 38. An output resistance in a low-frequency region corresponds to a resistance of the resistor 3. On the other hand, the output resistance in a high-frequency region in which the capacitor 5 appears to be a short-circuit is a parallel resistance of the resistors 3 and 4, thereby determining the gain. That is, (resistance of resistor 3)>(parallel resistance of resistors 3 and 4) is satisfied, so (gain in low-frequency region)>(gain in high-frequency region) is satisfied.
In the switching regulator as describe above, in order to maintain the stability of the output voltage to the oscillation, the resistance of the transconductance amplifier output resistor portion is reduced in the high-frequency region near the cutoff frequency of the LC filter, thereby reducing the gain.
However, the conventional switching regulator has the following problem. In order to prevent the abnormal oscillation, the gain of the transconductance amplifier error amplifier is reduced in the high-frequency region near the cutoff frequency of the LC filter. Then, even when a variation in output voltage occurs in the high-frequency region, a response of the transconductance amplifier error amplifier is slow, so a power supply voltage variation response characteristic is low.